This commit does the following:
- Splits the podman-systemd.unit.5.md into multiple files - one for each
quadlet file type, podman-quadlet.7.md for general quadlet information
and podman-quadlet-basic-usage.7.md for quadlet examples.
- Removes the original podman-systemd.unit.5.md file.
- Adds support for jinja2 templating language in the markdown_preprocess.
- Uses jinja2 in options/*.md to use the single .md file for both podman
subcommands man-pages and quadlet man-pages. This deduplicates
the Quadlet man-pages a lot.
- Adds new `@@option quadlet:source.md` preprocess command to import
such .md files from options directory.
Signed-off-by: Jan Kaluza <jkaluza@redhat.com>
